Photographic Image Quality:
Size (physical dimensions, with names for some sizes)
Bigger size is usually “better,” for reasons that we’ll discuss when we get to pixels below. Here are the four most common formats.
Full frame (FF), based on the traditional 35mm film camera, approximately 24×36mm, or 864mm². This format is found primarily in high-end professional cameras, but it also provides the point of reference for describing lens length in mm. (For example, a “normal” FF lens is 50mm; to find the same field of view on a smaller sensor, a “crop factor” or FOVCF is applied.)
APS-C, named after a long-defunct camera type that used standard 35mm film but shot images that were approximately half the standard size. There are several variations on this, but one common size is 23.67×15.7mm, or 370mm². These have an FOVCF of 1.5, meaning that a 35mm lens mounted on an APS-C body becomes the equivalent of a FF “normal” lens at 52.5mm. Many popular DSLR cameras use this format; APS-C (and even FF) sensors are now increasingly used in mirrorless system cameras, which provide a much smaller body while keeping the large-sensor image quality.
Micro four-thirds, 17.3×13mm, or 225mm². This format has been adopted by Olympus and Panasonic for their mirrorless system cameras. It is a compromise that permits cameras and lenses to be made smaller while still maintaining very good image quality. FOVCF is 2, thus a 25mm lens is “normal” in this format.
Compact camera sensors vary in size, but a popular one is 1/2.3 (the diagonal size in inches) or about 28mm², vastly smaller than the others with an FOVCF of 5.6! Confusingly, some cameras that are physically larger (sometimes known as “mega-zooms”) might still use a sensor this small. Buyer beware.
▸ Historical note: full-frame 35mm film was once the smallest photographic format, with most professional work done on 2¼" roll film or 4"×5" sheet film. Although there now are a very few digital camera systems available in a larger-than-full-frame size, they are highly specialized and expensive.
Pixel count (expressed in mega-pixels)
Mega-pixel (Mp) numbers, by themselves, are meaningless—one of the more misunderstood and abused terms in the camera business. Here’s why.
▸ Each sensor pixel is a discrete device that collects light energy, converts it to electrical energy, and reports how much it has collected to the camera’s computer chip—which turns it into a digital value between 0 and 255 (higher means more light or brighter). The final image file is simply a collection of all the pixel values, with some in-camera processing applied. (Actually, it’s much more complicated than this because the image is in color, but those details aren’t needed here.)
▸ The range of light values that human eyes can distinguish, from dark to light, is far greater than any sensor pixel can record. Any shadow value darker than the sensor can “see” is simply recorded as solid black, and any highlight value brighter than the sensor can accept is recorded as blank white. The physical size of the pixel determines how much light it can accept. Assuming the same underlying sensor technology, a larger pixel will always be able to record more gradations between light and dark than a smaller pixel can. With fewer gradations, the smaller pixel is more likely to report bogus values, seen as “noise” especially in darker areas of the final image.
▸ So more pixels are not always “better.” Given the same physical size sensor, more pixels do provide a sharper appearance to the image, but fewer pixels provide smoother light response. Given two different size sensors with the same pixel count, the larger one will always provide better response to light. The intended use of the image is also important. A 21-Mp full-frame file will produce astounding prints at 20"×30" or even 30"×45", but an image on the Internet can be produced from fewer Mp than any smart phone's built-in camera.